Cats get FIP when a common, usually harmless intestinal virus mutates inside their body into a deadly form that attacks their immune system. The vast majority of cats carry the original virus at some point in their lives, with 50 to 90% testing positive for antibodies. But only 5 to 10% of exposed cats ever develop FIP, and the transformation from routine gut infection to fatal disease depends on a chain of events involving viral mutation, the cat’s immune response, and often environmental stress.
It Starts With a Common Gut Virus
The story of FIP begins with feline enteric coronavirus, a widespread virus that infects the intestinal lining. Most cats pick it up as kittens, typically after 9 to 10 weeks of age, and it causes mild or no symptoms at all. Many cats shed the virus in their feces for weeks or months, then clear it. Others become long-term carriers that intermittently shed virus for years.
This intestinal coronavirus spreads through the fecal-oral route. A cat ingests tiny amounts of infected feces, usually from a shared litter box. The virus is remarkably easy to transmit. Research from UC Davis found it moves from room to room even in facilities with strict containment protocols, carried on shoes, clothing, hands, and cleaning supplies. Litter boxes and litter material are the primary vehicles, but any surface a caretaker touches can become a bridge between cats. This makes the virus almost impossible to keep out of multi-cat environments.
How the Virus Mutates Into FIP
FIP is not a separate virus that a cat “catches” from another cat. Instead, the harmless intestinal coronavirus mutates inside an individual cat’s body. The mutation changes the virus’s behavior in a critical way: instead of replicating quietly in intestinal cells, the altered virus gains the ability to infect white blood cells called macrophages. These immune cells travel throughout the body, and the virus essentially hijacks them as a transport system, spreading to organs like the liver, kidneys, brain, and eyes.
The mutation is random and unpredictable. A cat can carry the intestinal coronavirus for months or years before the mutation occurs, or it may never happen at all. Cornell’s Feline Health Center notes that FIP can develop weeks, months, or even years after a cat’s initial exposure to the coronavirus. There is no reliable way to predict which cats will experience the mutation and which won’t.
Why Only Some Cats Develop FIP
Since the intestinal coronavirus is nearly universal in multi-cat settings, the question isn’t really how cats get exposed. It’s why the virus mutates in some cats and not others. The answer involves at least three overlapping factors.
Immune Response
A cat’s immune system is the main line of defense. Cats with a strong cell-mediated immune response (the branch of the immune system that destroys infected cells directly) can often eliminate the mutated virus before it gains a foothold. Cats whose immune systems mount a strong antibody response but a weak cell-mediated response are more vulnerable. The antibodies can actually make things worse by forming immune complexes that damage blood vessels and organs, driving the inflammation that characterizes FIP.
Age
Young cats are far more susceptible, likely because their immune systems are still maturing. About 29% of FIP cases occur in kittens under six months old, and half of all cases are diagnosed before a cat’s first birthday. By age three, 80% of cases have already occurred. The incidence drops to extremely low levels between ages 7 and 11, though there is a small uptick of around 3% in elderly cats, possibly due to age-related immune decline.
Genetics
Certain breeds carry a higher genetic risk. Birman cats, for example, have a notably high incidence of FIP and are a highly inbred breed. Genomic studies have identified several regions in the Birman genome associated with FIP susceptibility, including genes involved in immune cell signaling and the body’s ability to process and present viral proteins to the immune system. Other purebred cats with limited genetic diversity also appear to be at elevated risk compared to mixed-breed cats, though no single gene has been identified as the definitive cause.
Stress as a Trigger
Stressful events are strongly associated with the onset of FIP, likely because stress hormones suppress immune function at exactly the wrong moment. Common triggers include rehoming or adoption (especially in kittens), spaying or neutering surgery, moving to a new household, introduction of new cats, and overcrowded living conditions. Shelters and catteries see disproportionately high rates of FIP partly because they combine high viral exposure with high stress levels in young, immunologically vulnerable cats.
This pattern explains why FIP so often appears shortly after a kitten is adopted. The kitten was likely exposed to the coronavirus in its birth environment or shelter, then the stress of rehoming and possibly surgery tipped the balance, allowing a mutation to take hold while the immune system was compromised.
Multi-Cat Homes Carry Higher Risk
The more cats sharing a space, the higher the viral load in the environment and the greater the chance of repeated exposure. Litter boxes are the epicenter. A cat shedding coronavirus contaminates the litter, and every other cat using that box gets a fresh dose. Even meticulous cleaning doesn’t eliminate the risk, because the virus can travel on fomites (hands, shoes, scoops, clothing) from one area to another.
Kittens born to infected mothers begin shedding the virus spontaneously around 9 to 10 weeks of age, which coincides roughly with the decline of protective maternal antibodies. This creates a window of vulnerability: the kitten starts encountering the virus just as its borrowed immune protection fades and its own immune system is still developing.
Single Cats Can Still Get FIP
While multi-cat environments pose the greatest risk, a cat living alone can also develop FIP. If the cat was exposed to the coronavirus at any point earlier in life, whether in a shelter, breeder, or previous home, that virus can persist in the gut and mutate later. FIP doesn’t require ongoing exposure to other infected cats. A single exposure early in life is enough to set the stage, and the mutation can occur years down the line.
What FIP Looks Like When It Develops
FIP takes two general forms. The “wet” form causes fluid to accumulate in the abdomen or chest, giving kittens a pot-bellied appearance or causing labored breathing. The “dry” form produces inflammatory lesions in organs like the eyes, brain, liver, and kidneys, leading to a wider range of symptoms: fever that doesn’t respond to antibiotics, weight loss, eye cloudiness, seizures, or jaundice. Many cats show a persistent, fluctuating fever and progressive weight loss as the earliest signs, along with lethargy and loss of appetite.
Both forms share one underlying process: the mutated virus triggers an overwhelming inflammatory response as the immune system attacks infected white blood cells throughout the body. The damage comes not just from the virus itself but from the cat’s own immune reaction to it.
Treatment Has Changed Dramatically
FIP was considered universally fatal until recently. Antiviral drugs originally developed for human coronavirus research have transformed the outlook. These medications work by blocking the virus’s ability to replicate inside cells, giving the immune system time to regain control. Treatment typically involves daily medication for 12 weeks, and many cats, particularly those diagnosed early, now achieve full remission. Access and cost vary by region, but the shift from a death sentence to a treatable disease represents one of the most significant advances in feline medicine in decades.